The instant invention concerns a corrosion and wear resistant ferrous base alloy composition which contains essential amounts of chromium, molybdenum, vanadium, and carbon, and optionally, silicon, manganese columbium and/or boron.
Numerous ferrous base alloy compositions are known and used in the fabrication of various types of bearings. Generally, such alloys are produced by conventional metal forming techniques, such as casting and hot working. However, when one attempts to utilize conventional procedures to produce a ferrous base alloy composition from the above enumerated ingredients, difficulties are often experienced.
One of the major impediments preventing the design in production of aircraft engines of increased engine performance and efficiency is the inability of the designs to operate at higher temperatures and speeds. This inability is caused by a lack of bearings which are capable of withstanding the softening effect and increased oxidation associated with high temperature.
It has been demonstrated that rolling contact fatigue life of bearings is significantly reduced when the hardness of the bearing components are reduced from hardness levels of Rockwell C60. When the hardness is compared with a bearing's dynamic capacity, as hardness drops from a ideal of 100 percent of capacity at Rockwell C of 64, the percentage of capacity drops below 90% as the hardness dips below 60. As hardness goes to as low as RC57 or lower, the capacity reaches only 50 percent of maximum. Clearly, the implications of this are that the candidate for a bearing alloy must exhibit high hardness and not only at room temperature but also at the high elevated temperatures which correspond to present and future operating conditions for engines.
While room temperature hardness of a bearing alloy is determined by the carbon content of the matrix, with at least 0.65 percent carbon being necessary to achieve adequate room temperature hardness, hot hardness is determined by other factors.
Another major cause for rejection and/or removal of aircraft engine bearings from service is the effect of corrosion. Corrosion results in significant increases in the aggregate bearing costs, reflecting both high maintenance costs and premature replacement costs.
Corrosion can result from inadequate preservation during storage and can also be caused by the corrosive effects associated with oil or lubricant contamination and/or degradation. Higher engine operating temperatures tend to increase the chemical degradation of either oil or lubricants thereby increasing the corrosive effect upon bearing components. It is a commonly accepted fact that corrosion resistance of bearing alloys is substantially increased by the addition of chromium, with a base threshold level of at least 12 percent chromium being necessary to provide effective corrosion resistance. Increase in the chromium level above 12 percent provides increased corrosion resistance.
Another important criterion for determining the acceptability of bearing material candidates is its ability to resist wear. This becomes of paramount importance when high-speed engine bearings operate, as they are subject to "fretting" wear. With every increasing tendencies towards higher engine speeds, bearings are now called to operate at rotational speeds which approach 3 million DN. This method of measuring rotational speed is calculated by multiplying the diameter D of the bore in mm by the rotation rate N in RPM. At these higher operating speeds, bearing toughness or resistance to crack propagation and fracture becomes increasingly more critical. Many exotic alloys and ceramic materials are resistant to corrosion and have adequate hardness but are brittle and fracture or do not wear well.
Accordingly, it is an object of the present invention to provide a corrosion and wear resistant ferrous base alloy composition which exhibits a high degree of hot hardness, toughness and wear resistance.
Another object of the invention is to provide a unique ferrous base alloy composition which is ideally suited as a material of construction for the fabrication of bearings which are intended for use at ambient, cryogenic or elevated temperatures.
These and other objects of the present invention will be apparent to those skilled in the art from a reading of the following specification and claims.